Reverse osmosis methods have been put to practical use as methods of concentrating a low-concentration aqueous solution of an organic substance with a membrane. However, these reverse osmosis methods necessitate the application of a pressure higher than the osmotic pressure of a separation solution to a separative membrane, so that the methods cannot be applied to a high-concentration aqueous solution owing to its high osmotic pressure. Thus, the solution separable by the reverse osmosis methods is limited in concentration. Meanwhile, pervaporation and vapor permeation have been noted as a new separating method free from influence of osmotic pressure. Pervaporation is methods characterized in that a substance to be separated is made to permeate a membrane in a gaseous state by feeding a separation solution to the primary side of the membrane, while reducing the pressure of the secondary side thereof or passing a carrier gas through the secondary side. On the other hand, vapor permeation is different from pervaporation in that a vapor mixture is fed to the primary side. The membrane-permeating substance can be recovered by condensing the permeating vapor under cooling. With respect to pervaporation, many studies have been made. For example, U.S. Pat. Nos. 3,750,735 and 4,067,805 disclose the separation of an organic substance from water with a polymer having an active anionic group, while U.S. Pat. Nos. 2,953,502 and 3,035,060 disclose the separation of ethanol from water with a cellulose acetate or polyvinyl alcohol membrane. Further, Japanese Patent Laid-Open No. 109,204/1984 discloses a cellulose acetate membrane and a polyvinyl alcohol membrane, while Japanese Patent Laid-Open No. 55,305/1984 discloses a crosslinked polyethyleneimine membrane. However, these membranes disclosed in the references are too poor in separation performance, particularly in permeation rate, to be put to practical use. Meanwhile, Japanese Patent Laid-Open No. 129,104/1985 discloses a membrane prepared from an anionic polysaccharide or a polysaccharide derivative as a membrane excellent in separation performance. However, this membrane has inherent unavoidable problems resulting from the nature of a natural high-molecular weight compound, for example, depolymerization with acid or alkali or decomposition with bacteria, so that the endurance and chemical resistance of the membrane fall short of expectations.
Generally, a hydrophobic separative membrane has disadvantages in that the surface thereof is hardly wettable and readily dries and that it tends to cause adsorption or clogging frequently. Therefore, for the purpose of overcoming these disadvantages, it has been attempted to impart a hydrophilic nature to such a membrane by various methods. For example, a method of adding a polyhydric alcohol such as polyethylene glycol or glycerin to a hydrophobic separative membrane has been generally adopted. However, the membrane thus improved in hydrophilic nature is disadvantageous in that the additives contained in the membrane are dissolved out into a filtrate in service. Further, Japanese Patent Publication No. 16187/1981 discloses a method which comprises applying a water-soluble polymer to a hydrophobic separative membrane by dipping and insolubilizing the applied water-soluble polymer by irradiation with electron beams, crystallization through heating or chemical reaction with formaldehyde or glyoxal to thereby impart a hydrophilic nature to the membrane. However, this method is disadvantageous in that it necessitates complicated operation so that it is unsuitable for practical use and that defects are often generated. Furthermore, Japanese Patent Laid-Open No. 35862/1983 discloses a method for imparting a hydrophilic nature to a hydrophobic porous filter membrane which comprises etching a polysulfone filter membrane in an atmosphere of vacuum electric discharge by sputtering. However, this method is disadvantageous in that the resulting porous filter membrane exhibits remarkably lowered mechanical strength.